In a wireless communication system, each mobile terminal must be able to establish communications (commonly referred to as registration) with a stationary base station in a cell. To allow this, some communication method must be defined and be known by all terminals. Most cellular communication systems in use today employ frequency division multiple access (FDMA). One type of FDMA uses busy-tone multiple access (BTMA). The digital equivalent of a BTMA uses either a repetitive bit or a repetitive field in a transmitted bit stream instead of a busy tone for inhibiting access, thus permitting the busy-tone channel to carry other data. The U.S. domestic cellular telephone systems, in accordance with EIA standard IS-3-D, employ BTMA and frequency re-use. Each cell is assigned one of a plurality of forward control channels designated as paging channels, paired in radio frequency with reverse control channels designated as access channels, and reused in such a way as to minimize interference between cells. Both the mobile terminals (conventional cellular telephones) and the base stations operate full-duplex. Repetitive bits called D-I bits positioned in the forward control channel message stream provide the busy-idle indication. The mobile terminal, prior to seeking an access channel, preselects which of the plurality of access channels it is receiving the best. The mobile terminal then determines its D-I status; if busy, it enters a random time-out to re-try, but if not busy, the terminal starts sending a message on the corresponding reverse control channel. Sometimes the reverse control channel in a first cell receives a message from a remote mobile terminal, the FM receiver of which is undesirably captured by a second cell using the same frequency pair. When this happens, the forward control channel of the first cell will raise its busy indication, and may actually attempt to exchange messages with the mobile terminal; but, because the mobile terminal's receiver is captured by a second cell, the protocol and control means ultimately protects the cellular system. Still, this effect reduces the capacity of the signaling channels. U.S. Pat. No. 5,047,762 of E. J. Bruckert proposes a scheme to help control this problem. A second fixed station receiving the remote mobile terminal's transmission uses the station identification and the signal strength of the remote mobile terminal to determine whether transmissions from other remote terminals should be inhibited. Depending upon the geographical proximity of the first fixed station to the second fixed station and the remote mobile terminal signal strength at the second fixed station, a signal representative of the remote mobile terminals signal strength at the second fixed station may be transmitted by the second fixed station. This representative signal may be used by a second remote mobile terminal to determine whether the second remote mobile terminal may transmit.
The U. S. patent application of J. P. Cotsonas, et al., Ser. No. 07/704516, filed on May 23, 1991, and assigned to the same assignee as the present application, discloses resolving contention for a dedicated channel among base units responding to a registration request from a mobile unit by assigning a time also referred to as time slot at which the base unit will respond as a function of the random number generated by the base unit. In addition, that U.S. patent application also teaches resolving contention by the base units responding at particular times based on the received signal strength from a mobile unit requesting the service to the individual base stations. The base unit receiving the greater received signal strength will respond first.
While the foregoing arrangements assist in avoiding contention between base stations in responding to a registration request from a mobile unit, the generation of a random number in order to determine a time slot suffers from the problem that there are a finite number of time slots available which increases the probability that two or more base units will generate a random number designating the same time slot. Further, the method of determining the time slot on the basis of the received signal strength also suffers from the problem that there will be locations where more than one base station receives the same signal strength from a mobile unit; hence, these base stations will be trying to respond in the same time slot.
In addition, the foregoing ,arrangements do not solve the problem of assigning a mobile unit to particular base station and giving that base station priority over the other base stations in responding to a registration request. This problem is an important one in personal communication services (PCS) systems where it is desirable for traffic control to assign mobile units to particular base stations and to utilize one of those base stations to service an assigned mobile unit if possible.